Phytohormone Abscisic Acid Research Articles

Polycomb repressive complex 2 attenuates ABA-induced senescence in Arabidopsis.

The phytohormone abscisic acid (ABA)-induced leaf senescence facilitates nutrient reuse and potentially contributes to enhancing plant stress tolerance. However, excessive senescence causes serious reductions in crop yield, and the mechanism by which senescence is finely tuned at different levels is still insufficiently understood. Here, we found that the double mutant of core enzymes of the polycomb repressive complex 2 (PRC2) is hypersensitive to ABA in Arabidopsis thaliana. To elucidate the interplay between ABA and PRC2 at the genome level, we extensively profiled the transcriptomic and epigenomic changes triggered by ABA. We observed that H3K27me3 preferentially targets ABA-induced senescence-associated genes (SAGs). In the double, but not single, mutant of PRC2 enzymes, these SAGs were derepressed and could be more highly induced by ABA compared with the wild-type, suggesting a redundant role for the PRC2 enzymes in negatively regulating ABA-induced senescence. Contrary to the rapid transcriptomic changes triggered by ABA, the reduction of H3K27me3 at these SAGs falls far behind the induction of their expression, indicating that PRC2-mediated H3K27me3 contributed to long-term damping of ABA-induced senescence to prevent an oversensitive response. The findings of this study may serve as a paradigm for a global understanding of the interplay between the rapid effects of a phytohormone such as ABA and the long-term effects of the epigenetic machinery in regulating plant senescence processes and environmental responses.

PYL8 mediates ABA perception in the root through non-cell-autonomous and ligand-stabilization–based mechanisms

The phytohormone abscisic acid (ABA) plays a key role regulating root growth, root system architecture, and root adaptive responses, such as hydrotropism. The molecular and cellular mechanisms that regulate the action of core ABA signaling components in roots are not fully understood. ABA is perceived through receptors from the PYR/PYL/RCAR family and PP2C coreceptors. PYL8/RCAR3 plays a nonredundant role in regulating primary and lateral root growth. Here we demonstrate that ABA specifically stabilizes PYL8 compared with other ABA receptors and induces accumulation of PYL8 in root nuclei. This requires ABA perception by PYL8 and leads to diminished ubiquitination of PYL8 in roots. The ABA agonist quinabactin, which promotes root ABA signaling through dimeric receptors, fails to stabilize the monomeric receptor PYL8. Moreover, a PYL8 mutant unable to bind ABA and inhibit PP2C is not stabilized by the ligand, whereas a PYL85KR mutant is more stable than PYL8 at endogenous ABA concentrations. The PYL8 transcript was detected in the epidermis and stele of the root meristem; however, the PYL8 protein was also detected in adjacent tissues. Expression of PYL8 driven by tissue-specific promoters revealed movement to adjacent tissues. Hence both inter- and intracellular trafficking of PYL8 appears to occur in the root apical meristem. Our findings reveal a non-cell-autonomous mechanism for hormone receptors and help explain the nonredundant role of PYL8-mediated root ABA signaling.

ZmOST1 mediates abscisic acid regulation of guard cell ion channels and drought stress responses

The phytohormone abscisic acid (ABA) is an important mediator in the drought response, participating in, among other processes, stomatal movements. In Arabidopsis thaliana, the serine/threonine protein kinase, OST1, regulates this response, but the function of its maize homolog has yet to be established. Here, we isolated ZmOST1 and show that its encoded protein indeed acts to regulate guard cell movement. ZmOST1 was ubiquitously expressed throughout the plant, being highly expressed in guard cells, and inducible both by exogenous ABA and water stress. Transient expression of a ZmOST1-GFP fusion protein, in maize mesophyll protoplasts, indicated its subcellular localization in the cytoplasm and nucleus. A Zmost1 loss-of-function mutant exhibited reduced sensitivity to ABA-activated slow anion channels in maize guard cells, and reduced drought tolerance. Constitutive expression of ZmOST1, in an A. thaliana ost1-1 mutant rescued the phenotype with respect both to the sensitivity of guard cell slow anion currents to ABA treatment and stomatal closure. Our findings indicate a positive regulatory role for ZmOST1 in guard cell ABA signaling and drought response in maize plants.

Abscisic and Jasmonic Acids Contribute to Soybean Tolerance to the Soybean Aphid (Aphis glycines Matsumura)

Plant resistance can provide effective, economical, and sustainable pest control. Tolerance to the soybean aphid has been identified and confirmed in the soybean KS4202. Although its resistance mechanisms are not fully understood, evidence suggests that enhanced detoxification of reactive oxygen species (ROS) is an active system under high aphid infestation. We further explored tolerance by evaluating the differences in constitutive and aphid-induced defenses in KS4202 through the expression of selected defense-related transcripts and the levels of the phytohormones abscisic acid (ABA), jasmonic acid (JA), JA-isoleucine (JA-Ile), cis-(+)-12-oxo-phytodienoic acid (OPDA), and salicylic acid (SA) over several time points. Higher constitutive levels of ABA and JA, and basal expression of ABA- and JA-related transcripts were found in the tolerant genotype. Conversely, aphid-induced defenses in KS4202 were expressed as an upregulation of peroxidases under prolonged aphid infestation (>7 days). Our results point at the importance of phytohormones in constitutive defense in KS4202 tolerance to the soybean aphid. Understanding the underlying mechanisms of tolerance will assist breeding for soybean with these traits, and perhaps help extend the durability of Rag (Resistance to Aphis glycines)-mediated resistance genes.

The influences of phytohormones on triacylglycerol accumulation in an oleaginous marine diatom Phaeodactylum tricornutum

Environmental stresses such as nitrate deprivation and high light are effective at increasing lipid content in microalgae, but they can also slow down and even stop growth. In this study, the phytohormones methyl jasmonate, salicylic acid, gibberellin, abscisic acid, and ethephon were introduced to cultures of the oleaginous marine diatom Phaeodactylum tricornutum in an attempt to increase growth and lipid production. Single-factor experiments showed that the influences of some of the phytohormones were closely related to their concentrations. Methyl jasmonate, abscisic acid, and salicylic acid promoted P. tricornutum growth and lipid accumulation at certain concentrations. The differing effects of the three phytohormones on P. tricornutum may be related to the respective phytohormone’s responsive cis-regulatory elements in the upstream regions of the triacylglycerol (TAG) synthesis genes. Methyl jasmonate, abscisic acid, and salicylic acid were further studied in response surface experiments, through which a 141% increase in TAG production was attained for 10-L cultures of P. tricornutum grown under optimal conditions. This study suggests that some phytohormones can promote P. tricornutum lipid accumulation without hindering growth. It also provides another strategy for improving the production of microalgae for use as biodiesel.

DNA demethylase ROS1 negatively regulates the imprinting of DOGL4 and seed dormancy in Arabidopsis thaliana.

Genomic imprinting is a form of epigenetic regulation resulting in differential gene expression that reflects the parent of origin. In plants, imprinted gene expression predominantly occurs in the seed endosperm. Maternal-specific DNA demethylation by the DNA demethylase DME frequently underlies genomic imprinting in endosperm. Whether other more ubiquitously expressed DNA demethylases regulate imprinting is unknown. Here, we found that the DNA demethylase ROS1 regulates the imprinting of DOGL4DOGL4 is expressed from the maternal allele in endosperm and displays preferential methylation and suppression of the paternal allele. We found that ROS1 negatively regulates imprinting by demethylating the paternal allele, preventing its hypermethylation and complete silencing. Furthermore, we found that DOGL4 negatively affects seed dormancy and response to the phytohormone abscisic acid and that ROS1 controls these processes by regulating DOGL4 Our results reveal roles for ROS1 in mitigating imprinted gene expression and regulating seed dormancy.

Correlations between Phytohormones and Drought Tolerance in Selected Brassica Crops: Chinese Cabbage, White Cabbage and Kale.

Drought is one of the major abiotic stresses affecting the productivity of Brassica crops. To understand the role of phytohormones in drought tolerance, we subjected Chinese cabbage (B. rapa ssp. pekinensis), white cabbage (B. oleracea var. capitata), and kale (B. oleracea var. acephala) to drought and examined the stress response on the physiological, biochemical and hormonal levels. The phytohormones abscisic acid (ABA), auxin indole-3-acetic acid (IAA), brassinosteroids (BRs), cytokinins (CKs), jasmonates (JAs), and salicylic acid (SA) were analyzed by ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS). Based on the physiological and biochemical markers the Chinese cabbage exhibited the lowest tolerance, followed by the white cabbage, while the kale appeared to be the most tolerant to drought. The drought tolerance of the kale correlated with increased levels of SA, ABA, IAA, CKs iP(R) and cZ(R), and typhasterol (TY), a precursor of active BRs. In contrast, the drought sensitivity of the Chinese cabbage correlated with a significant increase in ABA, JAs and the active BRs castasterol (CS) and brassinolide (BL). The moderately tolerant white cabbage, positioned between the kale and Chinese cabbage, showed more similarity in terms of the phytohormone patterns with the kale. We concluded that the drought tolerance in Brassicaceae is mostly determined by the increased endogenous levels of IAA, CKs, ABA and SA and the decreased levels of active BRs.

Unveiling Biosynthesis of the Phytohormone Abscisic Acid in Fungi: Unprecedented Mechanism of Core Scaffold Formation Catalyzed by an Unusual Sesquiterpene Synthase.

Abscisic acid (ABA) is a well-known phytohormone that regulates abiotic stresses. ABA produced by fungi is also proposed to be a virulence factor of fungal pathogens. Although its biosynthetic pathway in fungi was proposed by a series of feeding experiments, the enzyme catalyzing the reaction from farnesyl diphosphate to α-ionylideneethane remains to be identified. In this work, we identified the novel type of sesquiterpene synthase BcABA3 and its unprecedented three-step reaction mechanism involving two neutral intermediates, β-farnesene and allofarnesene. Database searches showed that BcABA3 has no homology with typical sesquiterpene synthases and that the homologous enzyme genes are found in more than 100 bacteria, suggesting that these enzymes form a new family of sesquiterpene synthases.

Characterization of a respiratory burst oxidase homolog from Gracilariopsis lemaneiformis (Rhodophyta) during stress and phytohormone treatments

Abstract Plant respiratory burst oxidase homologs (rbohs), a source of reactive oxygen species (ROS), play a central role in the response to biotic and abiotic stresses in plants. Here, an rboh gene from the seaweed Gracilariopsis lemaneiformis (Glrboh) was characterized and analyzed in terms of its structure and changes in its expression profile in response to high salinity, high temperature and phytohormone treatments. The results show that high salinity and high temperature mostly induced Glrboh expression at the mRNA, protein and enzyme activity levels within 3–6 h, and the levels decreased thereafter, but there was an almost continuous decline in Glrboh mRNA during the 24 h following exposure to high temperature. Under heat stress, abscisic acid (ABA) dramatically enhanced the levels of Glrboh mRNA at 3 h and increased Glrboh protein expression and enzyme activity throughout most of the 24-h period. However, salicylic acid (SA) and methyl jasmonate (MJ) had only slight and varied effects on Glrboh expression. These results indicate that Glrboh is involved in heat and salt stress responses and that the phytohormone ABA is more closely related to ROS production in this alga than SA and MJ.

Fast on-fiber derivatization and GC/MS analysis of phytohormones in wheat based on pencil-type coated carbon fibers

New coated carbon fibers (CCFs) have been synthesized, characterized and used as solid phase microextraction (SPME) matrix for the analysis of phytohormones (jasmonic acid, indole-3-acetic acid, and abscisic acid) in wheat samples. The SPME device, realized inserting CCFs in a pencil-type device, when coupled with gas chromatography-mass spectrometry, provides in few steps high recovery values (79–112%), fast on-fiber derivatization (30 s), good method reproducibility (RSD < 20%), low detection limits (0.5–2.1 ng g−1). The pencil-type CCFs-SPME device was successfully employed for the determination of phytohormone in wheat samples, allowing simple and quick extraction/derivatization/injection processes. The proposed device can be then considered as a promising and functional tool for fast and reliable extraction and preconcentration of analytes from real samples, allowing a simple derivatization procedure and direct injection in the chromatographic instrumentation.

Insights into the correlation between Physiological changes in and seed development of tartary buckwheat (Fagopyrum tataricum Gaertn.)

BackgroundTartary buckwheat (Fagopyrum tataricum Gaertn.) is a widely cultivated medicinal and edible crop with excellent economic and nutritional value. The development of tartary buckwheat seeds is a very complex process involving many expression-dependent physiological changes and regulation of a large number of genes and phytohormones. In recent years, the gene regulatory network governing the physiological changes occurring during seed development have received little attention.ResultsHere, we characterized the seed development of tartary buckwheat using light and electron microscopy and measured phytohormone and nutrient accumulation by using high performance liquid chromatography (HPLC) and by profiling the expression of key genes using RNA sequencing with the support of the tartary buckwheat genome. We first divided the development of tartary buckwheat seed into five stages that include complex changes in development, morphology, physiology and phytohormone levels. At the same time, the contents of phytohormones (gibberellin, indole-3-acetic acid, abscisic acid, and zeatin) and nutrients (rutin, starch, total proteins and soluble sugars) at five stages were determined, and their accumulation patterns in the development of tartary buckwheat seeds were analyzed. Second, gene expression patterns of tartary buckwheat samples were compared during three seed developmental stages (13, 19, and 25 days postanthesis, DPA), and 9 765 differentially expressed genes (DEGs) were identified. We analyzed the overlapping DEGs in different sample combinations and measured 665 DEGs in the three samples. Furthermore, expression patterns of DEGs related to phytohormones, flavonoids, starch, and storage proteins were analyzed. Third, we noted the correlation between the trait (physiological changes, nutrient changes) and metabolites during seed development, and discussed the key genes that might be involved in the synthesis and degradation of each of them.ConclusionWe provided abundant genomic resources for tartary buckwheat and Polygonaceae communities and revealed novel molecular insights into the correlations between the physiological changes and seed development of tartary buckwheat.

Apple bZIP transcription factor MdbZIP44 regulates abscisic acid-promoted anthocyanin accumulation.

Phytohormone abscisic acid (ABA) induces anthocyanin biosynthesis; however, the underlying molecular mechanism is less known. In this study, we found that the apple MYB transcription factor MdMYB1 activated anthocyanin biosynthesis in response to ABA. Using a yeast screening technique, we isolated MdbZIP44, an ABA-induced bZIP transcription factor in apple, as a co-partner with MdMYB1. MdbZIP44 promoted anthocyanin accumulation in response to ABA by enhancing the binding of MdMYB1 to the promoters of downstream target genes. Furthermore, we identified MdBT2, a BTB protein, as an MdbZIP44-interacting protein. A series of molecular, biochemical, and genetic analysis suggested that MdBT2 degraded MdbZIP44 protein through the Ubiquitin-26S proteasome system, thus inhibiting MdbZIP44-modulated anthocyanin biosynthesis. Taken together, we reveal a novel working mechanism of MdbZIP44-mediated anthocyanin biosynthesis in response to ABA.

Role of Phytohormones in Piriformospora indica-Induced Growth Promotion and Stress Tolerance in Plants: More Questions Than Answers.

Phytohormones play vital roles in the growth and development of plants as well as in interactions of plants with microbes such as endophytic fungi. The endophytic root-colonizing fungus Piriformospora indica promotes plant growth and performance, increases resistance of colonized plants to pathogens, insects and abiotic stress. Here, we discuss the roles of the phytohormones (auxins, cytokinin, gibberellins, abscisic acid, ethylene, salicylic acid, jasmonates, and brassinosteroids) in the interaction of P. indica with higher plant species, and compare available data with those from other (beneficial) microorganisms interacting with roots. Crosstalks between different hormones in balancing the plant responses to microbial signals is an emerging topic in current research. Furthermore, phytohormones play crucial roles in systemic signal propagation as well as interplant communication. P. indica interferes with plant hormone synthesis and signaling to stimulate growth, flowering time, differentiation and local and systemic immune responses. Plants adjust their hormone levels in the roots in response to the microbes to control colonization and fungal propagation. The available information on the roles of phytohormones in beneficial root–microbe interactions opens new questions of how P. indica manipulates the plant hormone metabolism to promote the benefits for both partners in the symbiosis.

Exogenous 24-Epibrassinolide alleviates oxidative damage from copper stress in grape (Vitis vinifera L.) cuttings

Copper (Cu) stress is the most common abiotic stress experienced in vineyards owing to the copper-based fungicides application. Plant hormones, including 24-Epibrassinolide (EBR), may alleviate the adverse impacts of heavy metal stress on plants. We investigated the effects of EBR pretreatment on root morphological parameters, active oxygen metabolism, osmolytes contents, antioxidant enzyme activity, endogenous phytohormone contents, and ascorbate-glutathione (AsA-GSH) cycle activity of one-year-old grape (Vitis vinifera L.) cuttings under Cu stress. Pretreatment with EBR significantly enhanced root morphological parameters (total root length, root surface area, root diameter, root volume, and tip number), increased soluble protein and proline contents, and significantly decreased the contents of H2O2, O2⋅−, and malondialdehyde (MDA) in roots and leaves. EBR pretreatment increased the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase oxidase (POD), and the contents of the endogenous phytohormones abscisic acid, jasmonic acid, and salicylic acid in the leaves. In addition, EBR regulated the balance of the AsA-GSH cycle by increasing the activities of monodehydroascorbate reductase (MDHAR), glutathione peroxidase (GR), ascorbate peroxidase (APX), and dehydroascorbate reductase (DHAR), and the contents of the antioxidant ascorbate (AsA) and dehydroascorbic acid (DHA), but the contents of glutathione (GSH) and oxidized glutathione (GSSG) decreased. Among the treatments tested, pretreatment with 0.10 mg/L EBR showed the optimal performance for alleviation of Cu toxicity. The results show that exogenous brassinosteroids reduce oxidative damage and improve the tolerance of Cu stress of grapevine cuttings.

The Ubiquitin E3 Ligase RHA2b Promotes Degradation of MYB30 in Abscisic Acid Signaling.

The phytohormone abscisic acid (ABA) is critical for plants encountering abiotic stress. We reported previously that the Arabidopsis (Arabidopsis thaliana) transcription factor MYB30 participates in ABA responses via SUMO ligase SAP-MIZ Domain-Containing SIZ1-mediated sumoylation. Here, we show that the RING-type ubiquitin E3 ligase RHA2b, which positively regulates ABA signaling, interacted with and ubiquitinated MYB30 to modulate MYB30 stability through the 26S proteasome pathway. The degradation rate of MYB30 was repressed significantly in the rha2b-1 mutant. Phenotypic analyses showed that MYB30 acts genetically downstream of RHA2b in ABA signaling. Substitutions of lysine-283 (K283) and K165 blocked ubiquitination, suggesting that these residues are sites of ubiquitination. K283 residue substitution significantly inhibited the degradation of MYB30 induced by ABA. The K165 site functioned additively with K283 in ABA-induced MYB30 degradation and ABA responses. At the same time, sumoylation protected MYB30 from degradation under cycloheximide and ABA treatment. Compared with MYB30, overexpression of MYB30-SUMO1 partially recovered the ABA sensitivity of siz1-2 But MYB30-SUMO1 exhibited similar localization with MYB30 in nuclei. Overall, our results suggest that RHA2b targets MYB30 for degradation to modulate ABA signaling. Considering that the K283 residue also is the major site for sumoylation, we propose that sumoylation and ubiquitination act antagonistically in the ABA response to regulate the stability of MYB30 by occupying the same residue.

Modulation of Phytohormone Signaling: A Primary Function of Flavonoids in Plant-Environment Interactions.

The old observation that plants preferentially synthesize flavonoids with respect to the wide range of phenylpropanoid structures when exposed to high doses of UV-B radiation has supported the view that flavonoids are primarily involved in absorbing the shortest solar wavelengths in photoprotection. However, there is compelling evidence that the biosynthesis of flavonoids is similarly upregulated in response to high photosynthetically active radiation in the presence or in the absence of UV-radiation, as well as in response to excess metal ions and photosynthetic redox unbalance. This supports the hypothesis that flavonoids may play prominent roles as scavengers of reactive oxygen species (ROS) generated by light excess. These ‘antioxidant’ functions of flavonoids appears robust, as maintained between different life kingdoms, e.g., plants and animals. The ability of flavonoids to buffer stress-induced large alterations in ROS homeostasis and, hence, to modulate the ROS-signaling cascade, is at the base of well-known functions of flavonoids as developmental regulators in both plants and animals. There is both long and very recent evidence indeed that, in plants, flavonoids may strongly affect phytohormone signaling, e.g., auxin and abscisic acid signaling. This function is served by flavonoids in a very low (nM) concentration range and involves the ability of flavonoids to inhibit the activity of a wide range of protein kinases, including but not limited to mitogen-activated protein kinases, that operate downstream of ROS in the regulation of cell growth and differentiation. For example, flavonoids inhibit the transport of auxin acting on serine–threonine PINOID (PID) kinases that regulate the localization of auxin efflux facilitators PIN-formed (PIN) proteins. Flavonoids may also determine auxin gradients at cellular and tissue levels, and the consequential developmental processes, by reducing auxin catabolism. Recent observations lead to the hypothesis that regulation/modulation of auxin transport/signaling is likely an ancestral function of flavonoids. The antagonistic functions of flavonoids on ABA-induced stomatal closure also offer novel hypotheses on the functional role of flavonoids in plant–environment interactions, in early as well as in modern terrestrial plants. Here, we surmise that the regulation of phytohormone signaling might have represented a primary function served by flavonols for the conquest of land by plants and it is still of major significance for the successful acclimation of modern terrestrial plants to a severe excess of radiant energy.

Increased Expression of 9-Cis-Epoxycarotenoid Dioxygenase, PtNCED1, Associated With Inhibited Seed Germination in a Terrestrial Orchid, Phaius tankervilliae.

The phytohormone abscisic acid (ABA) is involved in regulating seed dormancy and germination. A crucial step of ABA biosynthesis in higher plants is the oxidative cleavage of cis-epoxycarotenoids by 9-cis-epoxycarotenoid dioxygenase (NCED). Seed development in orchids is unusual because the embryos are minute in size, without obvious histodifferentiation, and lack endosperm. To understand the regulation of ABA biosynthesis in orchid seeds, we isolated and characterized a full-length cDNA encoding an NCED homolog, PtNCED1, from developing seeds of an ornamental orchid, Phaius tankervilliae. Germination percentage was high at 90 days after pollination (DAP), when a globular embryo proper with a degenerating suspensor was evident. After 90 DAP, seed maturation was accompanied by a decrease in water content and a concomitant increase in ABA content and PtNCED1 mRNA level along with a marked decrease in germination percentage. Mature seeds pretreated with NaOCl solution lowered ABA content and improved seed germination. Moreover, after seed germination, developing protocorms could respond to dehydration stress. Dehydration of protocorms stimulated an increase in PtNCED1 level along with ABA content. Our results provide evidence of the involvement of PtNCED1 in regulating endogenous ABA content in developing seeds and protocorms. The accumulation of endogenous ABA content in orchid seeds may have a critical role in seed dormancy and the protocorm response to water stress after seed germination.

Below versus above Ground Plant Sources of Abscisic Acid (ABA) at the Heart of Tropical Forest Response to Warming.

Warming surface temperatures and increasing frequency and duration of widespread droughts threaten the health of natural forests and agricultural crops. High temperatures (HT) and intense droughts can lead to the excessive plant water loss and the accumulation of reactive oxygen species (ROS) resulting in extensive physical and oxidative damage to sensitive plant components including photosynthetic membranes. ROS signaling is tightly integrated with signaling mechanisms of the potent phytohormone abscisic acid (ABA), which stimulates stomatal closure leading to a reduction in transpiration and net photosynthesis, alters hydraulic conductivities, and activates defense gene expression including antioxidant systems. While generally assumed to be produced in roots and transported to shoots following drought stress, recent evidence suggests that a large fraction of plant ABA is produced in leaves via the isoprenoid pathway. Thus, through stomatal regulation and stress signaling which alters water and carbon fluxes, we highlight the fact that ABA lies at the heart of the Carbon-Water-ROS Nexus of plant response to HT and drought stress. We discuss the current state of knowledge of ABA biosynthesis, transport, and degradation and the role of ABA and other isoprenoids in the oxidative stress response. We discuss potential variations in ABA production and stomatal sensitivity among different plant functional types including isohydric/anisohydric and pioneer/climax tree species. We describe experiments that would demonstrate the possibility of a direct energetic and carbon link between leaf ABA biosynthesis and photosynthesis, and discuss the potential for a positive feedback between leaf warming and enhanced ABA production together with reduced stomatal conductance and transpiration. Finally, we propose a new modeling framework to capture these interactions. We conclude by discussing the importance of ABA in diverse tropical ecosystems through increases in the thermotolerance of photosynthesis to drought and heat stress, and the global importance of these mechanisms to carbon and water cycling under climate change scenarios.

Leaf stage-associated resistance is correlated with phytohormones in a pathosystem-dependent manner.

It has been reported in several pathosystems that disease resistance can vary in leaves at different stages. However, how general this leaf stage-associated resistance is, and the molecular mechanism(s) underlying it, remain largely unknown. Here, we investigated the effect of leaf stage on basal resistance, effector-triggered immunity (ETI) and nonhost resistance, using eight pathosystems involving the hosts Arabidopsis thaliana, Nicotiana tabacum, and N. benthamiana and the pathogens Sclerotinia sclerotiorum, Pseudomonas syringae pv. tabaci, P. syringae pv. tomato DC3000, and Xanthomonas oryzae pv. oryzae (Xoo). We show evidence that leaf stage-associated resistance exists ubiquitously in plants, but with varying intensity at different stages in diverse pathosystems. Microarray expression profiling assays demonstrated that hundreds of genes involved in defense responses, phytohormone biosynthesis and signaling, and calcium signaling, were differentially expressed between leaves at different stages. The Arabidopsis mutants sid1, sid2-3, ein2, jar1-1, aba1 and aao3 lost leaf stage-associated resistance to S. sclerotiorum, and the mutants aba1 and sid2-3 were affected in leaf stage-associated RPS2/AvrRpt2+ -conferred ETI, whereas only the mutant sid2-3 influenced leaf stage-associated nonhost resistance to Xoo. Our results reveal that the phytohormones salicylic acid, ethylene, jasmonic acid and abscisic acid likely play an essential, but pathosystem-dependent, role in leaf stage-associated resistance.

The Flowering Repressor SVP Confers Drought Resistance in Arabidopsis by Regulating Abscisic Acid Catabolism

Terrestrial plants must cope with drought stress to survive. Under drought stress, plants accumulate the phytohormone abscisic acid (ABA) by increasing its biosynthesis and decreasing its catabolism. However, the regulatory pathways controlling ABA catabolism in response to drought remain largely unclear. Here, we report that the flowering repressor SHORT VEGETATIVE PHASE (SVP) is induced by drought stress and associates with the promoter regions of the ABA catabolism pathway genes CYP707A1, CYP707A3 and AtBG1, causing decreased expression of CYP707A1 and CYP707A3 but enhanced expression of AtBG1 in Arabidopsis leaves. Loss-of-function mutations in CYP707A1 and CYP707A3 or overexpression of AtBG1 could rescue the drought-hypersensitive phenotype of svp mutant plants by increasing cellular ABA levels. Collectively, our results suggest that SVP is a central regulator of ABA catabolism and that a regulatory pathway involving SVP, CYP707A1/3, and AtBG1 plays a critical role in plant response to water deficit and plant drought resistance.